Ignition kill switches have been implemented in many ways. Each one is usually different, making it a custom and effective form of security. Questions that people usually have about kill switches are: how should they be wired, and where should they be located?
While there are all kinds of places in a car's electrical system where a switch can be placed to disable the vehicle, I recommed wiring into the starting system. There are various interlocks in this system that cause the starter to not activate if all conditions are not correct. The best thing to do is to buy a service manual for your car and look at the schematic diagram of the starting system. This should give you some ideas about where to strategically place a switch in order to disarm the car.
Where should a switch be located? Almost anyplace. The custom factor of kill-switches makes them very effective. I once read a piece of journalism about these ex-car thieves who the journalists hired to try to break into some cars. The thieves were professionals, so they knew about all the tricks for getting past various name-brand alarm systems, the Club, etc. It didn't take them much time at all to defeat these types of security. But then they discovered that a car had a kill switch somewhere, and they just gave up. They said that they had to give up because kill switches are all different, it could take them a long time to find someone's switch and disarm it, time which they don't have in a public parking lot. So it makes more sense for them to give up on a kill-switch-armed car and move on to another car which has security systems they're more familiar with.
If you're interested in unique ideas for kill-switches, here are a few: